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Measuring Earthquake Magnitude

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Earth Polygraph
Teacher Contributed

Earth’s Polygraph

Topic

Seismographs

Student Exploration

Have you ever caught a friend lying to you about what they were doing and wanted them to take a lie detector test?

Just as a polygraph captures your friend’s nervous twitches, a seismograph captures the Earth’s tremors.

http://www.xkcd.com/711/

http://imgs.xkcd.com/comics/seismograph.png

How can seismographs give us insight into the Earth’s secrets? What is the Earth not telling us?

Extension Investigation

  1. Seismographs help seismologists determine the magnitude of an earthquake. By measuring the amount of time between when the S and P wave arrive and the amplitude of the wave, seismologists can calculate the magnitude. (http://hvo.wr.usgs.gov/volcanowatch/archive/other/magnitude.html) Take a look at the following map of current earthquakes: Identify http://rev.seis.sc.edu/earthquakes.html Identify one earthquake that you want to look more closely at. Where is it located? When did this earthquake occur? What was its magnitude?
  2. Another secret that seismographs help us to uncover is the epicenter of an earthquake. Here is a fun cartoon describing this process: Here http://earthguide.ucsd.edu/mystery_detectives/teach/epicenter/ia_trilateration_entire.html Here is a real world example of an earthquake that occurred in the Gulf of California on April 12, 2012: Do http://www.iris.edu/hq/files/programs/education_and_outreach/retm/tm_120412_gulfcalifornia/Baja_120412__EarthquakeTectonics.mov Do you want to try it for yourself? Use the same earthquake that you already identified to identify the S and P waves, their arrival times, and the speeds of the seismic waves!
    1. Return to the Rapid Earthquake Viewer site and click on the earthquake that you had identified earlier. (http://rev.seis.sc.edu/earthquakes.html)
    2. On the right hand side, there is seismograph data from multiple seismograph stations around the world as they received the seismic waves that were released from your earthquake. Identify 3 seismograph stations that you are going to focus on.
    3. Within each station’s line of data, you can identify when the P-wave arrived and when the S-wave arrived at that station. The arrival times of seismic waves should increase as you move farther away from the earthquake’s epicenter. Identify when the Pand S waves arrive at 3 of the seismograph stations. (If you are struggling to differentiate between the S and P waves, click on that particular station and check the box that overlays the estimated times.)
    4. Using the times and distances from the epicenter, plot your S-wave data and P-wave data as a line graph (with distance on the y-axis and time on the x-axis). Calculate the speeds (\text{Distance} = \text{Rate} \times \text{Time}) of the S and P waves.
  3. For further exploration, recent earthquakes explained by IRIS can be found here: http://www.iris.edu/hq/retm
  4. Want to make your own seismograph? http://cse.ssl.berkeley.edu/lessons/indiv/davis/hs/Seismograph.html

Resources Cited

XKCD. http://www.xkcd.com/711/

Hawaiian Volcano Observatory. USGS. http://hvo.wr.usgs.gov/volcanowatch/archive/other/magnitude.html

Rapid Earthquake Viewer (REV). http://rev.seis.sc.edu/earthquakes.html

Scripps Institution of Oceanography. University of California at San Diego. http://earthguide.ucsd.edu/mystery_detectives/teach/epicenter/ia_trilateration_entire.html

Incorporated Research Institutions for Seismology (IRIS). http://www.iris.edu/hq/files/programs/education_and_outreach/retm/tm_120412_gulfcalifornia/Baja_120412__EarthquakeTectonics.mov

http://www.iris.edu/hq/retm

Center for Science Education. Regents of the University of California. http://cse.ssl.berkeley.edu/lessons/indiv/davis/hs/Seismograph.html

Connections to other CK-12 Subject Areas

Earth Science

  • Seismic Waves
  • Earth’s Interior Material
  • Earth’s Layers
  • Earth’s Crust
  • Earth’s Mantle
  • Earth’s Core

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